BAGHDAD — Fears of a revival of sectarian killings in Baghdad soared Tuesday after the bodies of a Sunni Muslim prayer leader and two of his assistants were found in a morgue, four days after they were allegedly kidnapped by Shiite militiamen.

The Muslim Scholars Association said in a statement that Imam Nihad al-Jibouri and his two aides were abducted Thursday by men wearing the uniforms of government security forces in the mixed Baghdad neighborhood of Saidiyah, which was a site of widespread sectarian bloodletting during Iraq’s 2005-07 civil war. The association described the deaths as a brutal “execution” and said the bodies were discovered Monday.

The statement came as government forces battled swiftly moving Islamist insurgents in several locations across Iraq on Tuesday, while the United States weighed its options to help prop up the Baghdad government and authorities reportedly shut down the country’s largest refinery as a protective measure.

Jibouri and his two aides were abducted by “sectarian militias wearing the uniforms of government security forces, in full sight of the Maliki forces filling the streets of the district,” the statement said, referring to Prime Minister Nouri al-Maliki.

The murders recalled the brutal sectarian killings that raged through Baghdad throughout the middle of the last decade, as U.S. troops struggled to tame a flaming insurgency. The lightning assault by the al-Qaeda-inspired group, Islamic State of Iraq and the Levant (ISIL), in which large swaths of northern and western Iraq have fallen under insurgent control, has already engulfed much of the rest of the country in bloodletting.

The violence continued unchecked Tuesday, with the insurgents continuing to bear down on Baghdad from a number of northern locations. Government troops battled to hold off an ISIL assault against Baqubah, where there were reports that 44 prisoners were summarily executed, purportedly by Iraqi police before they retreated from a police station.

There was no independent confirmation of the reports. In Baghdad, a spokesman for the security forces, Saad Maan, told a news conference that Iraqi security forces had “preemptively” killed 65 unspecified “terrorists,” but he gave no further details.

Reports of mass killings have been emerging the confused, zig-zagging battlefields around the country as government forces attempt to recover from their humiliating rout a week ago, Shiite militias join in the fray and the militants continue to try to seize territory.

Baghdad has remained relatively calm, and the security forces appear to be in full control. But the appearance of thousands of irregular armed Shiite volunteers, who have responded to a call to arms from religious leaders and the government, has raised fears among many Sunnis that they may start to exact revenge for the killings of Shiite members of the security forces in the north.

“This is not the first incident, and it will not be the last,” said Hamid al-Mutlaq, a member of a political bloc headed by secular leader Ayad Allawi. “It’s not worse than usual yet, but it is getting worse as a result of sectarian sentiments and the influence of Iran.”

The statement of the Muslim Scholars Association, a Sunni religious organization that the U.S. military long suspected of involvement in the insurgency against U.S. troops, warned that there would be revenge.

“These crimes won’t go unpunished,” it said. “The day will come when we punish all the criminals and those who stand behind them.”

As fresh fighting raged north of Baghdad, authorities shut down the Baiji oil refinery about 140 miles northwest of the capital and evacuated its foreign staff, news services reported.

“Due to the recent attacks of militants by mortars, the refinery administration decided to evacuate foreign workers for their safety and also to completely shut down production units to avoid extensive damage that could result,” Reuters news agency quoted a chief engineer at the refinery as saying. He said there is sufficient gas oil, gasoline and kerosene to meet more than a month of domestic demand. The refinery was shut down overnight, Reuters said.

In the southern city of Basra, Iraq’s main port and oil hub, Turkey evacuated its consulate Tuesday, citing an increased security risk, Reuters reported. Staffers were transferred to neighboring Kuwait, Turkish Foreign Minister Ahmet Davutoglu wrote on his Twitter account.

A Foreign Ministry official in Ankara said the 18 staff members, including the consul general, would return to Turkey from Kuwait but that there were no plans to evacuate the Turkish Embassy in Baghdad, Reuters said.

Up to 275 American military personnel were being sent to Iraq to provide support and extra security at the heavily fortified U.S. Embassy in Baghdad, President Obama told Congress on Monday evening.

But the president is also considering “every option that is available,” including U.S. airstrikes, to help Baghdad counter the stunning offensive by ISIL, which Secretary of State John F. Kerry said Monday was threatening the stability of the entire Middle East.

Heavy fighting between government forces and rebels was reported Tuesday in Baqubah, on the northeastern approaches to Baghdad, according to the British Broadcasting Corp. The BBC reported that rebels took over several neighborhoods in the town about 35 miles from the Iraqi capital and captured the main police station. Government forces were also trying to recapture the town of Tal Afar, which fell to the insurgents Monday, the BBC reported.

Three police officers said the Baqubah station, which has a small jail, came under attack by Islamist militants in Diyala province on Monday night. The attackers tried to free the detainees, all suspected Sunni militants, the Associated Press reported. The police officers said the Shiite militiamen killed the detainees at close range.

The United Nations said the insurgents have almost certainly committed war crimes in their drive for power.

Besides weighing its options, the United States has also taken the highly unusual step of having its diplomats engage with diplomats from its longtime adversary, Iran, to discuss possible cooperation to help stop the Islamist insurgents’ lightning advance. The White House has ruled out the possibility of military cooperation with Tehran, however.

Nechirvan Barzani, leader of Iraq’s semiautonomous Kurdish region, said in an interview Tuesday with the BBC that he no longer believes Iraq can stay together after Sunni militants took over the key city of Mosul last week.

A week of brutal, intense fighting in Iraq has driven roughly half a million people from their homes and fanned fears of a bloody new civil war less than three years after the departure of U.S. troops.

“This is a severe and dangerous developing humanitarian emergency,” Nora Love, the Iraq country director of the International Rescue Committee, said Monday.

U.N. human rights chief Navi Pillay said in Geneva that the Sunni jihadists have carried out an “apparently systematic series of cold-blooded executions” near the northern city of Tikrit in recent days that “almost certainly amount to war crimes.”

She said that according to corroborated reports from various sources, hundreds of noncombatants had been executed, including police officers and soldiers who had surrendered or been captured, according to a statement from her office.

In addition, she said that, according to information received by U.N. employees in Iraq, the imam of the Grand Mosque in Mosul was killed for refusing to pledge allegiance to ISIL. The United Nations has received reports that a dozen local prayer leaders were executed under similar circumstances in front of Mosul’s al-Israa Mosque, she said.

On Sunday, ISIL posted gruesome photos online that appeared to show the mass execution of prisoners in the central Iraqi province of Salahuddin, north of Baghdad. The pictures caused outrage and raised fears that supporters of the Shiite-dominated government might take revenge on Sunnis, leading to a sectarian bloodbath.

The rebels have overrun a large swath of western and northern Iraq, which they are seeking to combine with areas they control in neighboring Syria. On Monday, an official in the Kurdish region in northern Iraq told journalists that the insurgents had seized two major airports, three airstrips and 30 military bases, including four that American forces once used.

At the briefing, Jabar Yawar Manda, general secretary of the Ministry of the Pesh Merga — or Kurdish military forces — used a laser pointer to draw broad circles over the center of a map of Iraq. The area is in the hands of ISIL and its allies, he said.

“All the airports are controlled by Daash,” he said, using the Arabic acronym for ISIL. “All the money is controlled by Daash. The weapons are all under their control — from the tanks to the AK-47s.”

The insurgents have threatened to extend their fight to Baghdad.

Obama said Monday that up to 275 U.S. military personnel will be sent to Iraq. “This force is deploying for the purpose of protecting U.S. citizens and property, if necessary, and is equipped for combat,” he said in a letter of notification to lawmakers under the War Powers Resolution.

The formal move comes after the administration deployed more than 50 Marines over the weekend to help protect the embassy. The State Department said it had moved some of the more than 5,000 embassy personnel to safer locations in Iraq and to Jordan. The embassy remains open.

Obama, who returned from a trip to California on Monday afternoon, held an early-evening White House meeting with members of his National Security Council to discuss options on Iraq that he had asked them to develop last week. Attendees included Vice President Biden, Secretary of State Kerry, Defense Secretary Chuck Hagel, national security adviser Susan E. Rice, Attorney General Eric H. Holder Jr. and top military officials.

ISIL expands control

Local news media reported clashes Monday between Iraq’s government forces and the jihadists in several areas north, east and west of the capital, including along Iraq’s border with Syria. Reuters reported that ISIL fighters and allied Sunni tribesmen overran the town of Saqlawiyah west of Baghdad, seizing six Humvees and two tanks.

The Associated Press quoted Iraqi security officials as saying that an army helicopter was shot down during fighting near the western city of Fallujah, killing the two-man crew. Insurgents also ambushed a vehicle carrying off-duty soldiers to Samarra, a town about 80 miles northwest of Baghdad, killing six and wounding four others, according to the news agency.

Fighting also was reported in Romanah, a village near a main border crossing into Syria in Iraq’s western Anbar province.

An Interior Ministry spokesman said Monday that Iraqi security forces had killed 56 “terrorists” and wounded 21 in operations just outside Baghdad in the previous 24 hours, AP reported.

Tens of thousands of Iraqi soldiers have fled the insurgent onslaught in the past week, some stripping off their uniforms and boots as they retreated.

ISIL’s biggest prize has been Mosul, the largest city in northern Iraq. One resident of the city, who gave his name as only Abu Zakariya, said in an interview that “a lot of people” from the area have joined the insurgents. He referred to them as “revolutionaries.”

At least three explosions believed to be airstrikes rattled the city overnight Sunday, Abu Zakariya said.

“There are people getting killed — these are revolutionaries and the Islamic State,” he said, referring to ISIL. “We hear about martyrs, but we can’t confirm.”

Kurdish officials at the briefing Monday said the discriminatory practices and violent crackdowns on Sunni protesters over the past year by Prime Minister Nouri al-Maliki’s administration were among the factors behind the jihadists’ current success.

ISIL forces on Sunday took control of Tal Afar, a key city 30 miles west of Mosul. By Monday, at least 3,000 residents had reached the desert town of Sinjar near the Syrian border, said Love, of the Washington-based International Rescue Committee.

With ISIL expanding its control through northern Iraq, many of those fleeing have become trapped with no clear escape route. “People are frightened and confused — some have walked up to four days to reach Dohuk to escape the violence,” Love said, referring to a Kurdish city north of Mosul.

In an interview with Yahoo News, Kerry said U.S. drone strikes and airstrikes are among the options to combat ISIL.

“When you have people murdering, assassinating in these mass massacres, you have to stop that. And you do what you need to do if you need to try to stop it from the air or otherwise,” he said.

Kerry also spoke with Yahoo’s Katie Couric about the possibility of collaborating with the Iranian government to curb advances by ISIL in Iraq.

But the Pentagon played down the possibility of military cooperation with the Islamic republic.

Iranian and American diplomats meeting Monday at a previously scheduled session in Vienna discussed the possibility of cooperation on Iraq, a State Department official said.

Loveday Morris in Irbil and Daniela Deane in London contributed to this report.